Field of the Invention
The present invention relates to a friction stir welding method, and more particularly to a method of advantageously joining together members formed of materials which are considered difficult to be joined together by the friction stwelding method, such as a cast aluminum alloy and 2000 series, 4000 series, 5000 series and 7000 series aluminum alloys, by the friction stir welding method using a rotary tool such as a bobbin tool and a self-reacting tool, which has two shoulder members and which is configured to perform a friction stir welding operation so that the members to be joined together are interposed between the two shoulder members, and a pressure is applied to those members through the two shoulder members.
Description of Related Art
A friction stir welding (FSW) method has been recently proposed in JP-T-7-505090, for example. The friction stir welding method is performed by inserting a rotary tool (specifically, a probe or a pin) into portions of two members to be joined together while rotating the rotary tool, to stir materials of those members and force the materials to flow by utilizing a friction heat, whereby those members are joined together in solid states without their fusion. A thermal strain is not likely to be generated by the friction stir welding method, since an amount of heat generated in an area welded by the friction stir welding method is smaller than an amount of heat generated in an area welded by a conventional fusion welding method. Further, the friction stir welding method permits joining of the members in the solid states, without phase transition which is characteristic of the fusion welding method in which the members to be joined together are molten and then solidified. For the above-described reasons, the friction stir welding method permits formation of a sound welded area. Accordingly, the friction stir welding method has been attracting attention, and employed in wider fields, particularly for joining together members formed of aluminum materials. The friction stir welding method greatly contributes to an increase of applications of the aluminum materials which are frontrunner materials that permit weight reduction. For instance, the aluminum materials can be used for structural members of transportation vehicles such as an automobile and a railway car, and housings of IT equipment, by performing the friction stir welding method for production of the structural members and housings.
By the way, the rotary tool used in the friction stir welding method described above generally has a structure in which a protrusion which is called the probe and which has a predetermined length is provided in a central part of a distal end portion of a cylindrical shoulder member. The friction stir welding method is performed by inserting a distal end of the probe into portions of the members to be joined together while rotating the shoulder member of the rotary tool about its axis, and joining together those members along a line or at a spot while applying a pressure to the above-described portions through a shoulder surface of the shoulder member, as disclosed in JP-T-7-505090 described above. However, in the friction stir welding method using the rotary tool constructed as described above, it is necessary to dispose a backing plate such as a surface plate on the back side of the portions which are to be joined together and into which the probe is to be inserted, so that a pressing force applied by the rotary tool to the above-described portions during the joining operation is received by the backing plate. Otherwise, the members to be joined together would be deformed by the pressure applied by the rotary tool, giving rise to a problem of failure of joining of those members. Therefore, it has been difficult to employ the friction stir welding method for joining of hollow members, since it is difficult to dispose the backing plate with respect to the hollow members.
Under the above-described circumstances, there has been proposed a rotary tool which does not require the use of the backing plate. The rotary tool has a cylindrical first shoulder member, and a second shoulder member which is provided at the distal end of the probe projecting from the first shoulder member, and which is opposed to the first shoulder member. The rotary tool is configured to perform a friction stir welding operation by simultaneously applying a pressure to both of the front and back surfaces of the portions of the members to be joined together, through the respective two shoulder members described above. For instance, there has been disclosed a friction stir welding method using a so-called bobbin tool having a structure in which the two shoulder members are fixedly connected with each other through the probe, in JP-A-2003-154471, for example. Further, JP-A-2003-181654 and JP-A-2009-18312, for example, propose so-called self-reacting tools in which one of the two shoulder members is configured so as to be movable with respect to the probe in its axial direction, so that a distance between the two shoulder members can be changed. By using the rotary tools described above, the hollow members and the like can be joined together without using the backing plate, whereby applications of the friction stir welding method have been further increased.
By the way, among aluminum members to be joined together, members formed of a cast aluminum alloy and 2000 series, 4000 series, 5000 series and 7000 series aluminum alloys according to JIS are considered difficult to be joined together by the friction stir welding method, since metals of those members are difficult to be friction-stirred, and are not sufficiently forced to flow by a friction-stirring action. In the case where the above-indicated aluminum members are joined together by the friction stir welding method by using the rotary tool having the two shoulder members, such as the bobbin tool and self-reacting tool described above, the metals of those aluminum members tend to stick to the circumferential surface of the probe of the rotary tool. Accordingly, when the friction stir welding operation is terminated and the rotary tool is removed from a welded area, the metals adhere or firmly stick to the circumferential surface of the probe of the rotary tool, giving rise to a problem that an extra work is required to remove the metals, and the rotary tool cannot be used for a subsequent friction stir welding operation, right after the last friction stir welding operation. Further, the metals of the aluminum members adhering to the rotary tool are removed from portions of the aluminum members where the rotary tool is removed from the welded area, resulting in formation of a large unwelded area in the above-described portions. Accordingly, it is necessary to cut off end portions of the members joined together on the side of termination of the welding operation, by a large amount, resulting in a loss of the materials.